Duplicating materials which utilize the photosensitivity of diazo compounds are common because they are inexpensive. Such materials can be classified into three main groups as follows.
The first is known as a wet development type duplicating material in which a photosensitive layer comprising a diazo compound and a coupler as main components is provided on a support. In this technique, an original is superposed on the material, exposed to light, and then developed using an alkaline solution.
The second is known as a dry development type duplicating material which differs from a wet development type in that ammonia gas is used for development instead of an alkaline solution.
The third is known as a heat development type duplicating material, which includes those materials containing an ammonia gas generator in a photosensitive layer such as urea that generates ammonia on heating, and in addition, those materials containing an alkali salt of a compound in the photosensitive layer such as trichloroacetic acid that loses its acidic properties on heating. Also known is a heat development type duplicating material containing a higher fatty acid amide as an coloring aid that melts on heating to facilitate the reaction between the diazo compound and the coupler.
The wet development type duplicating material is disadvantageous in that replenishment of the developing solution and disposal of the waste solution therefrom are undesirable. Also, this type of duplicating material involves troublesome maintenance and management due to the need for a relatively large apparatus. Additionally, it is difficult to retouch the material which is wet with the developing solution immediately after copying. Also, the copied images have poor durability upon long-term storage.
The dry development type duplicating material also is disadvantageous in that it requires, similarly to a wet development type material, replenishment of a developing solution, a gas-absorbing system for protecting the working atmosphere from ammonia gas that is generated in the development process and, consequently, a large apparatus. Furthermore, a strong odor of ammonia gas proliferates immediately after copying.
In contrast to these wet and dry development type duplicating materials, the heat development type duplicating material offers the advantage of easy maintenance because a developing solution is not used. However, the developing temperature thereof is as high as 150.degree. C. to 200.degree. C., and in addition, the temperature must be maintained with accuracy within .+-.10.degree. C. in order to avoid underdevelopment or changes in color tone. Therefore, to assure satisfactory images, the development apparatus is expensive. Furthermore, to endure such high developing temperatures, the diazo compounds used therein must have a greater heat resistance. However, such diazo compounds having a greater heat resistance are generally disadvantageous in obtaining high image density.
A number of proposals have been made to develop the material at lower temperatures (from 90.degree. C. to 130.degree. C.). However, these materials have a poor shelf life. Accordingly, a heat development type duplicating material is still not widely used in a diazo duplicating system, in spite of the advantage thereof in maintenance which is fully expected as compared with wet and dry development type materials.
On the other hand, the demand for the duplicating materials has increasingly diversified. For example, in addition to conventional materials which form a color image on a white background area, there is also a need for selecting a color tone for both the image and background areas depending on the intended application. This is because drawings and notices which are prepared from conventional monotonous duplicating materials fail to attract the viewer's attention.
In order to obtain the desired color density by heating a material having a layer which is formed on a support and which comprises a diazo compound, a coupler and a coloring aid, the respective components on heating must rapidly fuse, diffuse, and react to form a color dye. When a material is designed so that an image can provide a sufficiently high density even by a low heat temperature, as a matter of course, the coloring reaction proceeds to some extent in the material even during a storage at room temperature to stain (color) the background area which is intrinsically white.
In a heat development type duplicating material having a photosensitive layer provided on a support and which comprises a diazo compound, a coupler and a coloring aid, the above described problem of background staining which become remarkable when the material is designed to provide a sufficiently high color density even by a low heat temperature has nearly been solved by encapsulating the diazo compound in a microcapsule (JP-A-59-91438; The term "JP-A" as used herein means an "unexamined published Japanese patent application").
Even in this case, however, those duplicating materials which are designed to provide high image densities still suffer from insufficient shelf life.